Acidic phospholipids, unsaturated fatty acids, and limited proteolysis mimic the effect of calmodulin on the purified erythrocyte Ca2+ - ATPase.

The purified Ca2+-pumping ATPase of human erythrocyte membranes (Niggli, V., Adunyah, E. S., Penniston, J. T., and Carafoli, E. (1981) J. Biol. Chem. 256, 395-401) can be stimulated, in the absence of calmodulin, by other treatments. 1. A variety of acidic phospholipids (phosphatidylserine, cardiolipin, phosphatidylinositol, and phosphatidic acid) stimulate the Vmax and decrease the Km (Ca2+) of the isolated enzyme to the same extent as calmodulin. Unsaturated fatty acids (oleic and linoleic acid) have the same effect as phospholipids but at lower concentrations. Neutral phospholipids (phosphatidylcholine, sphingomyelin, and phosphatidylethanolamine) have no effect on the enzyme. The minimal proportion of acidic phospholipids in the environment of the enzyme necessary for full stimulation is about 40%. 2. The isolated enzyme, after reconstitution in phosphatidylcholine liposomes in the absence of calmodulin, can be activated by limited proteolysis. The trypsinized enzyme has the same high Vmax and high affinity for Ca2+ of the enzyme in the presence of calmodulin.